The present invention relates to color changers for spray coating equipment, and in particular to a method of and apparatus for coupling a grounded color changer and associated supplies of conductive paint to electrostatic spraying equipment, while maintaining electrical isolation between a high voltage at the spraying equipment and the color changer and paint supplies.
Color changers for spray coating apparatus have application in industrial operations where articles are to be spray coated at a station or as they move along a production line. Where the articles are to be coated a wide variety of colors, it generally is not practical to establish separate spray stations or production lines for each color, or even to spray a long sequence of articles one color, then another long sequence a second color, etc. Instead, it is desirable to be able to make color changes rapidly and simply at a single station.
Electrostatic spray coating devices have an increased painting efficiency over nonelectrostatic types. When painting with an electrostatic spraying apparatus, it is necessary to have some means for applying a charge to the paint. In some apparatus, charging is accomplished by an electrode connected to a high voltage supply and placed in close proximity to or in contact with the paint either just prior or close to its point of atomization. In rotary atomization apparatus, the rotary atomizer is ordinarily made of a conductive material and connected to the power supply, so the atomizer itself is the electrode. Whichever type of apparatus is used, the charging potential is usually on the order of several tens of kilovolts, and the electrostatic charging process works well when spraying nonconductive paints. However, when spraying conductive paints, such as water based paints, precautions must be taken to prevent the high voltage at the spraying apparatus from shorting to ground through a conductive column of paint being delivered to the spraying apparatus.
One known approach to prevent shorting the high voltage to ground is to isolate the entire paint supply and color change system from ground potential. This allows the paint system to "float" at the charging potential, but has the drawback that a large amount of electrical energy is capacitively stored in the system. To prevent the capacitively stored energy from presenting a shock hazard to operating personnel, it is necessary to provide a protective enclosure around the color changer and paint supplies, which increases costs and requires that the spraying operation be shut down and the system electrically discharged whenever necessary to replenish the supplies of paint. Also, during operation of the system, the large amount of capactively stored energy increases the probability of arcing and the possibility of an explosion when volatile paints are sprayed.
Another approach, which can be used when the paint is relatively nonconductive, is to ground the paint supplies and color changer and connect the spraying apparatus to the color changer through a hose of sufficient length that the electrical resistance of the paint column in the hose is large enough to reduce current leakage through the paint column to a level that does not short out the charging voltage or cause it to fall to an unacceptably low level. A disadvantage of the approach is that the hose, due to its extended length, is hard to manage and difficult to clean during color changes. Also, while the extended length of the hose limits the magnitude of leakage current, some leakage nonetheless occurs and represents "wasted" charging energy. For relatively conductive coating materials, such as waterborne paints, the resistance of the paint is so low that the technique is not practical.